According to the National Bicycle Safety Education Curriculum, riding in the road isn't introduced until the child is between the ages of 9-12 years old. Part of the training for children ages 5-8 have them bicycling on the sidewalk but only after they have gone over the safety issues like crossing driveways, stopping at the end of sidewalks. There is obviously more involved in the training at each age and for those details you will want to look through the curriculum at the website stated above.
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